Brake conversion – am I on the right track?

[WDE_BDY]

Being an accountant I thought I would pick the brains of people who do this stuff in the real world to see if I have made any really obvious screwups in my thinking.

Background
My new wheels don’t fit over the factory KP61 Starlet brakes as the caliper sits past the face of the hub and the back of my wheels is very flat. Since I am fitting a 7K and KP rotors aren’t available anyway (or at least very hard to find) I thought it was a good opportunity to do an upgrade. The criteria I had were –
- Must fit my wheels (14x7 neg 17 offset Spitfires)
- Cost effective
- Un-modded consumables (rotors/pads) if possible
- Vented rotors
- Easily available parts
- Must mount to the rear of the Starlet hub
- Simple caliper bracket
- Not too heavy

These criteria put a few restrictions on what I could do, the wheels are the most limiting factor as there can only be minimal overhang of the caliper past the hub face and the 14” size limits the diameter. I looked at a lot different conversions on the net, most were no good as they mounted to the front of the hub. Most callipers also wouldn’t clear my wheels without setting the caliper well back which meant the rotor had to fit over the factory caliper mounts. Basically I had to find some low profile calipers or run a separate rotor hat and Wilwood or similar discs. The only real issue I have with this is I do some pretty high mileage so decided I didn’t really want race style parts.

So a session at the wreckers with a tape measure saw me pick up some calipers and discs from the wreckers, I think they are from a Ford Laser/Mazda Astina (based on the disc dimensions, only had engine and front suspension but definitely Mazda/Ford something). Discs were no good with out modifying (centre bore too small) but the single piston sliding caliper had a very low profile front face. Easy to find decent pads for as they a pretty common size. As a bonus the calipers are not much heavier than the factory ones.
Ford/Mazda Caliper Picture - Click Me

So with some serious measuring and an Excel based copy of the DBA catalogue I dug out some potential rotor options. Nothing was a bolt up fit, mainly due to the small PCD that the factory Starlet rotor bolts on with as well as having the thread in the rotors. Basically came down to a rotor from an E30 BMW which is 260x22mm, 100mm PCD and a 66mm centrebore. The total height of the rotor is 35mm which gives just enough room to get a caliper bracket on the front side of the strut (otherwise you need to machine the back of the caliper mounts flat), it will require the mount to be threaded though as there will not be enough rotor clearance to fit a bolt in. The caliper end will also need to be threaded to bolt the caliper up.
KP Rotor Picture - Click Me
BMW Rotor Picture - Click Me

So the key things I need to know is around the caliper bracket and fitting the rotor to the hub. For the bracket I am looking about 8 to 10mm thick, will this be thick enough to get enough thread into? When I measured up the factory mount that the caliper came off it had a thickness of about 8mm so I presume keeping the same thickness should be safe. The other question is would you just tap a thread in the flat plate or I have seem them done with threaded inserts that are welded to the plate.

For securing the rotor my first thought as a cheap bastard was just machine down the factory rotor, run longer bolts and use it to sandwich the rotor. Deciding not to be that dodgy I have planned on getting a ring made that centres the rotor on the back of the hub (hub 64mm vs rotor 66mm) and is threaded. BMW rotor would be redrilled to fit the PCD of the starlet hub and then sandwiched by the centering ring, seems safer than bolts and nuts as you don’t have the risk of a nut working its way loose.

Any feedback appreciated, have a used rotor on the way to test how good my measuring is and will dummy it up from there.

Callum

[oldcorollas]

just an aside, I know people have used a vented Camira disc (DBA021=235mm) with volvo(?) caliper, under 13's on Ke1X, but not sure how far outboard they sit in relation to hub face

if sticking with solid, do KE70 discs fit the starlet too?


there is also NSX rear disc (303mm) that might fit DBA487... not under 14's tho

from previous search for KE10 bits

going from a disc height of 36mm (30-42mm), and centre hole of 64mm..
235 = camira JD front DBA021 (centre hole 60)
260 = BMW E30 front DBA679 (centre hole 66)
303 = N$X rear disc DBA487 (centre hole 64)

[WDE_BDY]

Camira's and Volvo's are not so common here, NZ is the land of Jap imports. BMW discs are less than $100NZ a pair brand new, just need redrilling.
The KE70 discs fit I believe (overseas version apparently run one of the KE70 rotors) but not much of an upgrade in disc size with a lot of messing around with caliper mounts.

Callum

[oldcorollas]

making a backing ring for the BMW disc to centre it is good.
have also heard of people using the metal type Nylocks (forget what they are called, but have metal instead of nylon to lock) with no dramas.
as long as there is space behind the beemer disc, all good.
machining stick disc and using it with a centering ring would be all good as well.
you coudl also get the bore of the beemer disc opened up a bit, to allow a thicker ring to be used?

for the caliper mount, are you using factory backing plate, or making a whole new one?
from memory the original brackets are not very thick, but reasonably strong.
8mm seems a little thin, but if the metal is a good grade (from memory, Cruzida used a Bisalloy grade for caliper dogbones?), then should be ok, but as you say, easy to get a much larger nut/threaded chunk tacked onto the back to give greater thread length.


how expensive are replacement rotors for a separate hat/rotor setup? more then the bmw discs+ drilling/machining?

[WDE_BDY]

The Starlet has cast caliper mounts attached to the stub rather than a bolt on setup, so not as easy as some of the Toyota's to make a new bracket for.
Wilwood rotors seem to be around $100 each if you have the right contacts, none of the off the shelf hats seem suitable so would need custom made. Only a one off cost obviously but then the probably quite a bit more expensive than the planned centering ring. Preference is not to maching the centre of the disc if possible to make replacing discs relatively easy, I think the 1mm lip on the centering ring should be fine as it is only for locating and not taking any real load. One of the places I talked to about discs wanted about $15 to redrill the discs as it is pretty straight forward (if buying the discs from them).

Callum

[oldcorollas]

ahh bugger.. KE's have 4bolts on strut for caliper bracket..

could also get the hub machined down another mm to allow a thicker portion for the centering lip?

anyways, you are on the right track ..
hmm, no chance of swapping struts?

[WDE_BDY]

Nah, strut change will push the track out and nothing else actually bolts up without changing steering arms. Also have the dual height adjustable coilovers already builit...

Callum

[WDE_BDY]

So as a quick test measure had a couple of holes drilled and tapped in the BMW rotor that showed up yesterday, this is trial fitment step ONLY (not really enough meat in the disc for this).



http://img809.imageshack.us/img809/5712/bmwbrakes04.jpg
http://img836.imageshack.us/img836/3397/bmwbrakes05.jpg
http://img838.imageshack.us/img838/961/bmwbrakes06.jpg
http://img708.imageshack.us/img708/2663/bmwbrakes07.jpg
http://img228.imageshack.us/img228/848/bmwbrakes08.jpg
http://img580.imageshack.us/img580/7456/bmwbrakes09.jpg
http://img204.imageshack.us/img204/2069/bmwbrakes10.jpg
http://img805.imageshack.us/img805/2087/bmwbrakes11.jpg
Looking promising, as it is a sliding caliper I test fitted it with a set of new pads so the front face of the caliper is as far out as it will ever get (moves in as pads wear). It is close but the caliper just clears the wheel face, I may be able to get myself a few mm more with a clean up of the bac k of the wheel or even the front edge fo the caliper if I really need to. Has a casting mark that sits out on the caliper that to my untrained eye does nothing but will get a second opinion on that.
Now to dummy up a caliper mount, will need to be shaped a bit but doesn't look too tricky at this stage. Not totallly convinced I can get the bracket thick enough to thread especially at the strut end but could could use a counter sunk allen head bolt instead and then weld threaded inserts in at the caliper end. That wold work for a 5mm plate flat bracket, or start with thicker material threaded at the caliper end and machind down to 6 or 7mm at the strut end.

Callum

[WDE_BDY]

So I made me a caliper bracket, but it broke when I tried to to screw the bolt into it.





Next time I will make it out of something stronger
Bracket was dummied up in 10mm wood, with a little sanding to get it to fit at the strut end. Probably have about 7-8mm there, not really enough to be threaded but should be enough to recess a countersunk allen head bolt though. Caliper end will be 10-12mm thick, thinking threaded inserts welded in might be the best approach as to my non expert engineer mind machining a 12mm thick plate down to 7mm at the strut might introduce weak points that a flat plate wouldn't have.
Thinking of extending the bracket to meet up with two of the 3 brake backing plate bolt holes just as extra support. Open to any feedback anyone has.

Callum

[oldcorollas]

how much rim clearance will there be with caliper in that position?
no chance of moving disc a little further out?

[WDE_BDY]

Rims are the big problem, normally I would go rotor over and make life easy but no space with the offset I am running and a very flat rear face on the rim as well.

[oldcorollas]

fair enough, was just thinking an extra 2-3mm would make a big difference to the bracket, and being able to use pressed in studs (wheel studs) or somesuch..

extending to 3rd bolt will give it a fair bit more leverage and stability, and worth doing i reckon.

will botl heads hit the inner part of disc? can disc be hogged out a bit?

[WDE_BDY]

Bolt heads would get too close to the disc and not keen to mod the disc too much as then you need to do the same every time you replace a disc (do pretty reasonable mileage in this car). Looking at some of the bolt on Wilwood kits they tend to use the countersunk allen hean bolts so probably a suitable solution. I have a little more space where the bolts sits in relation to the disc so should be do-able.

Callum